This application claims the priority of German Patent Application No. 198 28 843.3, filed Jun. 27, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for manufacturing coated short fibers and to short fibers made by this method.
Short fibers are used in particular for manufacturing fiber-reinforced composites, for example fiber-reinforced ceramic matrix composites (CMC) and fiber-reinforced metal matrix composites (MMC). The fiber reinforcement causes increased ductility and hence increased tolerance of the composite matrix to damage. The fibers used for this purpose should in general be provided with a protective coating on all sides. This protective coating is deposited, of example, by chemical vapor deposition (CVD) or by dipping in a bath. The purpose of the coating is to prevent the fibers from reacting with the composite matrix (ceramic or metal). If possible, the fibers should be embedded in the composite unaltered and undamaged to achieve optimum ductility. Short fibers are particularly suitable for manufacturing fiber-reinforced composites since they become embedded in the composite in all three dimensions in disorderly fashion, resulting in a composite with isotropic material properties.
The technology of CVD coating and dip coating is also known. Thus far, however, only endless fibers have been coated, not short fibers. This is because the commercially available fibers are in the form of coated fiber bundles having a size to ensure that the fiber bundle holds together. Before CVD coating, the fiber bundle must be desized so that the fiber bundle comes apart and the fibers can be coated on all sides. This desizing is done by a thermal process in the case of endless fibers, in which the endless fibers are continuously fed from a feed roller, passed through an oven, then rolled up again onto a feed roller. In this way, the individual fibers resulting from desizing can be handled.
This is not possible with short fibers, however. Desizing causes the fiber bundle to disintegrate into disorderly individual fibers or filaments that cannot be readily handled. Thus, for manufacturing coated short fibers, endless fibers desized as described above are first coated then cut up into short fibers. Another alternative would be to coat the short fiber bundles directly (i.e., without desizing). The first alternative has the disadvantage that the cut surfaces are uncoated and therefore unprotected, so that they are able to react with the composite matrix and are exposed to oxidation. The second alternative would have the drawback that the sizing would interfere with the material properties of the resulting fiber-reinforced composite. Direct coating of desized short fibers has been repeatedly tested or proposed, but has not thus far been feasible.
Hence the goal of the present invention is to create a method of by which short fiber bundles can be desized and CVD-coated on all sides, whereby the short fiber bundles made by cutting up coated endless fibers can be coated on the cut surfaces.
According to the present invention, provision is also made for the short sized fiber bundles to be exposed to a high-frequency field in a reactor. When the short fiber bundle enters the high-frequency zone of the reactor, the coating is suddenly decomposed into gaseous products. The resulting gas phase also forces the individual fibers apart. Subsequently, the fibers thus separated are (1) exposed to at least one coating agent present in the gas phase, and (2) CVD-coated in the high-frequency field.
The method according to the present invention also makes it possible to coat the cut surfaces of short fibers made by cutting up coated endless fibers. These cut fibers are also CVD-coated in a high-frequency field with at least one of the coating agents present in the gas phase. This seals the cut surfaces so that they are unable to react with the composite matrix and are at the same time protected from oxidation.
The method according to the present invention thus makes it possible for the first time to coat short fibers on all sides directly. The method according to the present invention is not substrate-specific, so that fibers of all types can be coated.
The short fibers coated on all sides according to the present invention are protected against a reaction with the composite matrix and are thus chemically unchanged when they are embedded in the matrix. The coating acts as a diffusion and reaction barrier. The resulting composite is thus distinguished by improved mechanical properties, particularly increased ductility and accordingly enhanced strength and tolerance to damage. The coating also improves the wetability of the fiber surfaces by the matrix. This makes it possible to have a higher proportion of fiber components by volume in the composite, particularly with ceramic composites. Also, the individual fibers in the matrix are distributed more homogeneously than the coated fiber bundles previously used. When metal matrix composites are made by gas pressure melt infiltration, the infiltration pressure can be reduced to approximately one-tenth of the previous value because of the improved wetability.
Advantageously, carbon fibers, particularly short recyclate fibers, are used. Recyclate fibers, produced by shredding fiber-reinforced plastics, can be freed of the plastic matrix clinging to them by the method according to the present invention. A combination of high-frequency, ultrasound, and shock-wave treatment is particularly suitable for this purpose.
Another advantageous embodiment provides for the short fiber bundles coated with a size or a plastic matrix to first be mechanical loosened before they are placed in the high-frequency field. This facilitates the desizing that follows. Microwaves can advantageously be used as high-frequency waves.
The protective coating can be pyrocarbon, SiC, and/or Si coatings, but also TiN, TiC, and/or TiCN coatings. By the method according to the present invention, short carbon fibers can be coated with a pyrocarbon coating and with a carbon-graduated and/or Si-graduated silicon carbide coating or with a carbon-graduated and/or N-graduated titanium carbonitride coating. These graduated coatings are preferably deposited in a coating step by changing the deposition parameters.
For making silicon-containing coatings, methyltrichlorosilane/hydrogen, for example, may be used as the coating agent. For making titanium-containing coatings, titanium tetrachloride with nitrogen and possibly methane in hydrogen may be used as the coating agent.
The thickness of the coating can be controlled by varying the time the fibers spend in the reactor. This is advantageous, for example, when sealing the cut surfaces of short fibers made conventionally from coated endless fibers. In such cases, a thin coating suffices to seal off the cut surfaces.